The thinness of the semiconductor process rule makes it possible that a plurality of radio circuits are implemented in one semiconductor chip. As a result, the adoption of MIMO technology has become more popular in broadband wireless communication systems for the purpose of enhancing the capacity of system or improving the reliability of communications. For example, although most of wireless LAN system which has become more popular in a home and in a office are in conformity with IEEE802.11a or IEEE802.11g which are based on OFDM (Orthogonal Frequency Division Multiplexing) technique, the new standard of IEEE802.11n which is a new standard by use of MIMO technology is pushed forward. And recently, release of the product based on the MIMO technology in conformity with the draft of IEEE802.11n begins. The draft of IEEE802.11n is described in non-patent literature 1.
Further, the MIMO technique has been introduced in many standards of next generation wireless system such as Next Generation PHS and mobile WiMAX. About these systems, it is described in non-patent literature 2.
And, in the mobile system, it is expected that an MIMO technology will be introduced after super 3G (the name of coming generation mobile system of NTT DoCoMo) or ultra 3G (the name of coming generation mobile system of KDDI) with which next generation broadband service will be provided. Thus, it is expected that the MIMO technology prevails rapidly as broadband wireless communications become popular. This is because high throughput and reliable wireless communication can be realized by the use of MIMO technology with which space-multiplexing and transmission or reception diversity can be implemented.
According to Shannon's theorem based on information theory, throughput is limited mainly by the bandwidth of the transmission channel. With MIMO technology, channel capacity can be largely increased in case of high S/N in comparison with SISO. FIG. 41 is a diagram which shows enhancement effect of the channel capacity by use of MIMO transmission. In the figure, a solid line, a broken line, the alternate long and short dash line show the Shannon's channel capacity in case of the number of receive and transmit antennas is 4*4, 2*2, and 1*1, respectively.
As for the enhancement of the channel capacity by the use of MIMO technology, it can be realized by multi-stream transmission which transmits a plurality of data series independently from each other on the same location, in same frequency at the same time.
In the method of performing a multi-stream transfer, there are two schemes. One is the method transmitting a plurality of data series using eigenbeams which are orthogonal to each other. And the other is the method transmitting a plurality of data series and estimating the data series using techniques such as the pseudo inverse matrices in a receiving side. Information sharing of the channel matrix between the transmitter and receiver is necessary in the former scheme.
Besides multi-stream transmission, transmitting/receiving diversity can be performed by use of MIMO technology.
In case of the information of channel matrix is known in transmitting side, transmission diversity can be performed using the information in a transmitting side. Further, in case of the information of channel matrix is unknown in transmitting side, transmission diversity can be performed by use of the space-time code, too. In the case that the number of antennas in the receiving side is greater than that of spatial streams, reception diversity of equal-gain combining or maximum ratio combining can be performed by use of baseband processing.
In most of the wireless system which MIMO technology has been introduced, the number of antennas of hand-held terminal is one in general because of power consumption and battery life. In the case of this, high-speed transmission by use of multi-stream transfer cannot be achieved. However, even in this case, the channel capacity of the transfer between base-station and hand-held terminal can be increased by improving the mean S/N of the transfer by applying transmission diversity and the reception diversity by use of space-time code.
FIG. 42 is a block diagram which shows the wireless communication system using the conventional MIMO technology. In this wireless communication system, two-way wireless communication is performed between wireless communication device A and wireless communication device B.
Wireless communication device A includes P antennas of A1, A2, . . . , AP, P transmitter-receiver circuit 13-1, 13-2, . . . , 13-P, spatial mapping unit 14, space time block code coding/decoding unit 15, K baseband (BB) modulating/demodulating unit 16-1, 16-2, . . . , 16-k and stream parser 17. Herein, k is the number of the spatial streams. Wireless communication device B includes Q antennas of B1, B2, . . . , BQ, Q transmitter-receiver circuit 23-1, 23-2, . . . , 23-Q, spatial mapping unit 24, space-time block code coding/decoding unit 25, K baseband (BB) modulating/demodulating unit 26-1, 26-2, . . . , 26-k and stream parser 27.
First, in the case that channel matrix is unknown in the transmitting side and multi-stream transmission is performed between the wireless communication devices, data series of transmitting is divided into the data series of the k (k is a natural number more than or equal to 2) in stream parser 17 in wireless communication device of the transmitting side, e.g., wireless communication device A. Each divided data series is modulated by baseband modulating/demodulating unit 16-1, 16-2, . . . , 16-k, and K baseband modulating signal are generated. The baseband modulating signal is output to space-time block codes coding/decoding unit 15. In this case, because the space-time block code is not used in this system, the input signal of space-time block codes coding/decoding unit 15 is just output to spatial mapping unit 14. And in spatial mapping unit 14, direct mapping is applied. And finally, each baseband-modulating signal is provided to each transmitting/receiving circuit 13-1, 13-2, . . . , 13-k, and each baseband-modulating signal is converted into RF frequency and transmitted through each antenna A1, A2, . . . , AK. Note that, the transmission diversity can be performed by selecting K antennas with which good communication quality is obtained of P antennas and by using them. For example, in IEEE802.11n, antenna selection sequence for this antenna selection is determined.
The signals transmitted from the wireless communication device A are received in each antenna B1, B2, . . . , BQ, and down-converted to baseband received signals by each transmitting/receiving circuit 23-1, 23-2, . . . , 23-Q in the wireless communication device of the receiving side, e.g., wireless communication device B. The baseband received signals are provided to the spatial mapping unit 24 in which an estimation of the baseband received signals corresponding to each data series is performed by use of techniques such as the pseudo inverse matrices in which the estimated information of the channel matrix is used, and the spatial mapping unit 24 outputs the baseband received signals to the space-time block codes coding/decoding unit 25. In this case, because the space-time code is not used in this system, space-time block codes coding/decoding unit 25 just outputs an input signal to baseband modulating/demodulating unit 26-1, 26-2, . . . , 26-k. Herein, k is the number of the spatial streams. Baseband received signal is demodulated by baseband demodulation unit 26-1, 26-2, . . . , 26-k which output the decoded data of each data series. The decoded data from baseband demodulating unit 26-1, 26-2, . . . , 26-k are provided to the stream parser 27 which outputs the synthesized received data.
The estimation of channel matrix is performed by use of received signal of preamble. An example is provided as follows. Orthogonal series of Hadamard codes S is transmitted in preamble sequence from each antenna A1, A2, . . . , AP of wireless communication device A in the transmitting side. For example, in the case that the number of antennas is four, (1,1,1,1), (1,−1,1,−1), (1,1,−1,−1), (1,−1,−1,1) is transmitted from each antenna A1, A2, A3, A4, respectively. In the case that the number of antennas in the receiving side is four, reception series BT in each antenna B1, B2, B3, B4 corresponding to the preamble sequence can be expressed in a 4 by 4 matrix, and the estimation of the channel matrix H can be obtained by H=T·S−1. If Hadamard matrix is used, the above-mentioned calculation can be implemented just making addition and subtraction of received signal of preamble sequence.
Next, in the case that channel matrix is known in the transmitting side and multi-stream transfer is performed using eigenbeams, the transmitting data series is divided into K (K is a natural number of more than or equal to 2) data series in the stream parser 17 in the wireless communication device A. Each divided data series is modulated by baseband modulating/demodulating unit 16-1, 16-2, . . . , 16-k which output baseband-modulating signals. The baseband modulating signals are provided to the space-time block codes coding/decoding unit 15. In this case, the input signal of the space-time block codes coding/decoding unit 15 is just output to the spatial mapping unit 14, because space-time block code is not used.
In the spatial mapping unit 14, the baseband modulating signals of each data series are multiplied by K eigenvectors which can be calculated from known channel matrix. For example, in the case that the number of spatial streams K is 2 and the number of antennas is 4, two eigenvectors V1 (v11,v12,v13,v14) and V2 (v21,v22,v23,v24) are calculated by use of the preamble, and corresponding to two baseband modulating signal m1, m2, v11·m1+v21·m2, v12·m1+v22·m2, v13·m1+v23·m2, v14·m1+v24·m2 are calculated for each antenna, respectively, and theses signals are transmitted from each of the antenna. The output signals of spatial mapping unit 14 are provided to each transmitting/receiving circuit 13-1, 13-2, . . . , 13-4, and are up-converted into RF frequency and transmitted through antennas A1, A2, . . . , A4, respectively.
The signals transmitted from the wireless communication device A are received in each radio antenna B1, B2, . . . , B4, and down-converted to baseband received signals by each transmitting/receiving circuit 23-1, 23-2, . . . , 23-4 in the wireless communication device B. In the spatial mapping unit 24, the baseband received signals of each data series are multiplied by K eigenvectors which can be obtained from the channel matrix. For example, in the case that the number of the spatial streams k is two and the number of antennas is four, two eigenvectors V1′ (v11′,v12′,v13′,v14′) and V2′ (v21′,v22′,v23′,v24′) can be obtained by the reception of preamble, and four baseband received signals r1,r2,r3,r4, v11′·r1+v12′·r2+v13′·r3+v14′·r4 and v21′·r1+v22′·r2+v23′·r3+v24′ are calculated and provided to the space-time block codes coding/decoding unit 25. In this case, the space-time block codes coding/decoding unit 25 just outputs an input signal to the baseband modulating/demodulating unit 26-1, 26-2, . . . , 26-k, because space-time block code is not used.
The baseband received signal is demodulated by the baseband demodulation unit 26-1, 26-2, . . . , 26-k which output the decoded data of each data series. The decoded data from the baseband demodulation unit 26-1, 26-2, . . . , 26-k are provided to the Stream parser 27 which outputs the synthesized decoded data as the final received data.
Note that, there are two methods to obtain the channel matrix information in the transmitting side. One is a method estimating channel matrix using a preamble of the received signal from the opposite wireless communication device in the nearest past. And the other is a method receiving the feedback information on the channel matrix which is estimated at the opposite wireless communication device.
Next, in the case that the channel matrix is unknown in the transmitting side and a space-time code is used to perform transmit diversity, the transmitting data is divided into K data series (K is a natural number more than or equal to 1) in the stream parser 17 in the wireless communication device A. In case of k is one, transmitted data is just output to baseband modulating/demodulating unit 16-1. Each divided data series is modulated by the baseband modulating/demodulating unit 16-1, 16-2, . . . , 16-k which output the baseband modulating signals. The baseband modulating signal is provided to the space-time block codes coding/decoding unit 15.
In the space-time block codes coding/decoding unit 15, the block coding by use of space-time code is performed in every m symbols (m is a natural number more than or equal to 2). For example, for the hand-held terminal which uses 1-stream transfer, coding procedure of two antennas is performed for information data symbol. In this case, two information symbols (s0,s1) are transmitted in two-symbol period. In more detail, antenna 0 transmits signals (s0,−s1*) is transmitted from antenna 0, 1 antenna transmission symbol (s1,s0*), and the information of 2 symbols is transmitted during 2 symbol periods (coded rate 1). In this example, two series of space-time block code are provided by the space-time block codes coding/decoding unit 15. In spatial mapping unit 14, direct mapping is performed. In other words, the output signal of space-time block codes coding/decoding unit 15 is just output to transmitting/receiving circuit 13-1, 13-2, . . . , 13-L, and they are up-converted into RF frequency and transmitted through each antennas of A1, A2, . . . , AL. Herein, L is a natural numbers more than or equal to 2 which represents the number of series of the space-time block code.
The signals transmitted from the wireless communication device A are received in each antenna B1, B2, . . . , BQ, and downconverted to baseband received signal by each transmitting/receiving circuit 23-1, 23-2, . . . , 23-Q which are provided to the spatial mapping unit 24 in the wireless communication device B. In the spatial mapping unit 24, diversity combining is performed by use of Q input signals, and L synthesized baseband received signal can be obtained. In the case of Q=L, an input signal is just output to space-time block codes coding/decoding unit 25.
In the space-time block codes coding/decoding unit 25, the baseband received signal corresponding to each data series are calculated by use of the estimated channel matrix which are provided to the baseband modulating/demodulating unit 26-1, 26-2, . . . , 26-k. For example, for the hand-held terminal which uses 1-stream transfer, the space-time block code is decoded by use of numerical expression (1) concerning information symbol (s0,s1), reception symbol (r0,r1) and estimated symbol (s0,s1). Herein, X* represents a complex conjugate of X.
                    [                  EQUATION          ⁢                                          ⁢          1                ]                                                                      (                                                                      s                  0                                                                                                      s                  1                                                              )                =                              (                                                                                h                    0                    *                                                                                        h                    1                                                                                                                    h                    1                    *                                                                                        -                                          h                      0                                                                                            )                    ·                      (                                                                                r                    0                                                                                                                    r                    1                    *                                                                        )                                              (        1        )            
Baseband received signal is demodulated by baseband demodulation unit 26-1, 26-2, . . . , 26-k which output decoded data corresponding to each data series. The decoded data from baseband demodulation unit 26-1, 26-2, . . . , 26-k are provided to the Stream parser 27 which outputs the synthesized received data.
In the broadband wireless communication system based on these conventional MIMO technology, a high-throughput is realized by use of multi-level modulation such as 64QAM and 256QAM or by use of spatial mapping of the MIMO technology, the area where high-throughput is achieved is limited to the area near base-station in which S/N (Signal to noise power ratio) is very high.
Although it is possible to enlarge the area in which high-throughput transfer can be performed by use of both diversity combining and spatial multiplexing of MIMO technology, the diversity gain which is available in the high-throughput area is 5 dB extent at most. In the diversity gain of this extent, the great enhancement of the area where high-throughput transfer can be performed cannot be expected.
As an example, in FIG. 43, the simulation result on the relationship between the throughput and the distance from base-station in IEEE802.11n system. It is assumed that transmission power PT is 23 dBm and mean received signal power decay with the distance using 3.5-power rule in FIG. 43.
Solid line indicates the simulation result in which MIMO technology of 2 (transmitting)*4 (receiving) is used and spatial multiplexing (SDM) of 2-streams is performed. In this case, thick line indicates the simulation result of using 64QAM. In this case, PHY transmission rate is 108 Mbps and MAC maximum throughput is 78 Mbps as is shown in this figure. Thin line indicates the simulation result of 256QAM. In this case, PHY transmission rate is 144 Mbps and MAC maximum throughput is 92 Mbps as is shown in this figure. From this figure, it can be found that both PHY transmission rate and MAC maximum throughput of 256 QAM is greater than that of 64QAM, but radius of service area of 256QAM is smaller than that of 64QAM. The area in which throughput takes the value of almost 100 Mbps is limited to the surrounding area near AP and the range of the high-throughput transmission is around 10 m or less.
Broken line indicates the simulation result in which MIMO technology of 3 (transmitting)*4 (receiving) is used and spatial multiplexing (SDM) of 3-streams is performed. In this case, thick line indicates the simulation result of using 64QAM. In this case, PHY transmission rate is 162 Mbps and MAC maximum throughput is 100 Mbps as is shown in this figure. Thin line indicates the simulation result of 256QAM. In this case, PHY transmission rate is 216 Mbps and MAC maximum throughput is 118 Mbps as is shown in this figure. In this case, it can be found from this figure, that both PHY transmission rate and MAC maximum throughput of 256 QAM is greater than that of 64QAM, but radius of service area of 256QAM is smaller than that of 64QAM.
Next, Dash-dot indicates the simulation result in which MIMO technology of 4 (transmitting)*4 (receiving) is used and spatial multiplexing (SDM) of 2-streams is performed. In this case, thick line indicates the simulation result of using 64QAM. And thin line indicates the simulation result of using 256QAM. In these cases, the radius of service area is a little greater than that of the case in which spatial multiplexing is not used. However, the amount of this improvement is small and the area in which throughput takes the value of almost 100 Mbps is limited to the surrounding area near AP and the range of the high-throughput transmission is around 10 m or less in this case after all. A space-time code is a kind of the transmission diversity, and both PHY transmission rate and MAC maximum throughput are the same as that of the case using spatial multiplexing of 2-stream without STBC.
Dash-dot-dot indicates the simulation result in which MIMO technology of 4 (transmitting)*4 (receiving) is used and spatial multiplexing (SDM) of 3-streams and transmit antenna selecting diversity is performed. In this case, thick line indicates the simulation result of using 64QAM. And thin line indicates the simulation result of using 256QAM. In these cases, the radius of service area is a little greater than that of the case in which spatial multiplexing of 3-streams is performed but transmit antenna-selecting diversity is not performed. However, the amount of this improvement is still small and the area in which throughput takes the value of almost 100 Mbps is limited to the surrounding area near AP and the range of the high-throughput transmission is around 10 m or less in this case after all. The selection of transmit antenna is a kind of the transmission diversity, and PHY transmission rate and MAC maximum throughput are the same as that of the case using spatial multiplexing of 3-stream without TS.
The radius of service area of the wireless LAN system of IEEE802.11n is said to be around 100 m, however, the area where high-throughput transmission around 100 Mbps is achieved is limited to PAN (Personal Area Network) area in which the distance from AP is around 10 m or less.
There are several techniques which extend the service area of wireless LAN. For example, Xirrus Inc. (U.S.A.) has already developed the technology which extend the area of wireless LAN and base-stations based on this technique is already manufactured.
FIG. 44 is a block diagram showing the basic structure of the wireless base-station by use of the technique developed by of Xirrus. This wireless base-station includes 16 access points (AP), and each access point includes a directional antenna, respectively. The direction of the directional antennas of each access points differs from each other and directivity for all azimuths is provided with 16 directional antennas. Different channel of the radio frequency is assigned to each access point from each other. The wireless controller of Xirrus controls 16 access points and the directional antennas. In the case of the access point provided by Xirrus, the antenna gain of about 12 dB can be obtained in comparison with the case that an omni-antenna is used.
Other example of the technique which extends the service area of wireless LAN is provided by Ruckus Wireless Inc. (U.S.A.) and wireless base-station based on this technique is already manufactured. This technique is described in patent document 1.
FIG. 45 is a block diagram showing the structure of wireless base-station based on the technique of Ruckus Wireless Inc. The technology of Ruckus Wireless is the same as that of Xirrus in the point that antenna gain of 16 directional antennas is used to extend service area. However, the combination of active directional antenna is changed by use of high-frequency switch adaptively in the technology of Ruckus Wireless while a plurality of AP including a directional antenna is used in the technology of Xirrus. With this structure, there should be only one access point in a wireless base-station, therefore cost and power consumption power may be reduced.
With theses technology using directional antennas such as Xirrus or Ruckus Wireless in the AP, antenna gain of 12 dB can be obtained in transmission/reception of the AP at most, and as a result, the radius of the area where high-throughput transmission around 100 Mbps can be performed will be extended to around 2 times. However, a radius of the extended service area is only around 20 m, and the service area is still almost PAN rather than LAN.
Further, beam forming of MIMO technology can be used to extend communications area. This is the technique which forms an optimum beam based on channel matrix information both in transmitting side and in receiving side by setting diversity combining information to the signal of each antenna, and which extend the communications area. There are two kinds of beam forming techniques. One is a technique using eigenvectors. The other is a technique estimating the arrival direction of the received signals. Though the former can extend communications area greatly by suppressing multi-path fading, it is needed to feed back the information of channel matrix within a time much smaller than fading period. On the other hand, latter can form a beam based on the estimated arrival direction of received signals. The estimated value of arrival direction does not change rapidly, so fast feedback will not be required. However, the gain in transmitting/receiving signals is 12 dB extent at most, therefore great extension of the communication area will not be obtained.
As mentioned above, the beam forming technique is mandatory to achieve large amount of extension of the communications area. In other words, eigenbeam transmission is needed to expand communications area greatly. To obtain the diversity combining information corresponding to the signal of each antenna, a calculation of correlation matrix of the channel matrix should be done first in the conventional technique. And a calculation of eigenvector corresponding to the maximum eigenvalue of the correlation matrix is done next. And finally, the calculated eigenvectors are set as optimum diversity combining information for each antenna.
Here, the amount of arithmetic processing of the calculation of correlation matrix is proportional to the square of the number of the antennas, and the amount of arithmetic processing of the calculation of the eigenvector is proportional to the cube of the number of the antennas. As the number of antennas increases, the amount of arithmetic processing to find optimum diversity combining information increases rapidly. Therefore, there is the upper limit on the number of antennas.
In the case that maximum fading frequency is 50 Hz (corresponding to the velocity of 10 km/hour under the condition that carrier frequency is 5 GHz), the wireless communication device should transfer the information of channel matrix within several milliseconds in order to reduce estimation error of the channel matrix. Therefore the calculation of eigenvectors should have been completed in several milliseconds. In the case that the number of antennas is more than or equal to 5, the eigenvectors cannot be obtained arithmetically. In this case, the eigenvectors can be obtained by use of repetitive operation, and therefore the processing time for the calculation becomes critical.
Also, though huge transmission gain can be obtained by using an eigenbeam transfer of MIMO technology, transmission of the information of channel matrix is needed in order to share the information of channel matrix between the transmitter and the receiver of the transmission. As the number of the antennas increase, the amount of information of the channel matrix increases. In fast fading environment, a frequent transfer of the information of channel matrix is necessary, and this will cause deterioration of channel capacity of the system.
Considering the conditions as described above, the upper bound of the number of the antenna is around four. Practically, in many standard of wireless communication systems which use the most advanced MIMO technology such as IEEE802.11n and IEEE802.16, the maximum number of antennas is defined as four.
Further, in the conventional MIMO technology, diversity combining is performed by a baseband processing, and therefore the input signal of each transmitting/receiving circuit is a received signal which is not subjected to diversity combining. Here, there is no diversity gain or MIMO gain on the input signal of each transmitting/receiving circuit. As described above, the detection of received signal should be done in the transmitting/receiving circuit without the use of transmit beam forming in the state of initial acquisition of eigenbeam. Therefore, in the conventional MIMO technology, the communications area is limited to the area where eigenbeam can be formed without initial acquisition.
In order to realize the true broadband wireless LAN system with which high-throughput transfer of around 100 Mbps can be performed at any point in the service area, the performance of conventional MIMO technology which uses the combination of beam forming and spatial multiplexing is not enough. And there is no technology which provides enough performance of that.